(1) Field of the Invention
This invention relates to water soluble films. More particularly, this invention relates to water soluble films useful in packaging applications, especially for dry detergents and similar water soluble commodities, formed from homogeneous blends of water soluble polyvinyl alcohol and polyacrylic acid.
(2) Discussion of the Prior Art
Packaging materials formed from film-forming materials which are water soluble have been used for many years for packaging water soluble or dispersible dry, solid materials which may be toxic or otherwise harmful to the user or which may be difficult to weigh out in accurate portions, or simply for the convenience of the user. Examples of typical materials used in aqueous environments which have been packaged or proposed for packaging in water soluble films include, for example, cleaning products, such as laundry detergents, bleaches, and caustic cleansers; pesticides, such as herbicides, fungicides, insecticides, and nematocides which are applied as aqueous sprays; and various other pulverulent water soluble or dispersible chemicals, such as carbon black, pigments, dyes, etc; food products, and the like.
One of the most widely investigated water soluble film-forming materials used or promoted for packaging of such powdery materials is polyvinyl alcohol (i.e., partially hydrolyzed polyvinyl acetate). At hydrolysis levels of up to about 97-99%, polyvinyl alcohol is soluble in water although the rates of dissolution are too slow to be practical, especially in cold water.
Many attempts have been reported in the patent literature to improve upon the properties of water soluble polyvinyl alcohol packaging films. These efforts have included, for example, selection of special plasticizers, e.g., U.S. Pat. No. 2,948,697--J. A. Robertson and U.S. Pat. No. 3,106,543--J. N. Milne, assigned to E. I. duPont de Nemours; U.S. Pat. No. 3,157,611--M. K. Lindemann, assigned to Air Reduction Co.; U.S. Pat. No. 3,374,195--T. S. Bianco, et al., assigned to Mono-Sol Div. of Baldwin-Montrose Chemical Co.; modification or copolymerization of the polyvinyl alcohol, e.g., U.S. Pat. No. 3,300,546--R. L. Baechtold, assigned to American Cyanamid, U.S. Pat. Nos. 3,441,547 and 3,505,303--M. K. Lindemann, assigned to Air Reduction Co., U.S. Pat. No. 3,277,009--M. Freifeld, et al., assigned to General Airline & Film Corp.; and blending polyvinyl alcohol with other polymers, e.g., U.S. Pat. No. 2,850,741--I. M. Klein, U.S. Pat. Nos. 3,695,989 and 3,892,905--R. E. Albert, assigned to E. I. duPont de Nemours.
However, none of these efforts have been entirely satisfactory in providing films which are sufficiently stable against degradation of physical properties when stored under relatively low humidity/low temperature or relatively high humidity/high temperature conditions. Thus, at low humidity/low temperature conditions, many of the films become brittle and lose elasticity, resulting in loss of impact resistance and tear strength. The may also lose their clarity and heat sealability. At high humidity/high temperature conditions (e.g., 100.degree. F. and 80% R.H.) the degree of hydrolysis of the polyvinyl alcohol may increase to levels at which the polyvinyl alcohol is no longer water soluble or the film may simply become too soft and tacky to be stored or handled. The films may become water insoluble after only 2 weeks at these conditions. For this reason, it is usually necessary to store the films under specially controlled atmospheric conditions or to provide special waterproof protective release films between plies of the water soluble polyvinyl alcohol films, as shown, for example, Albert--U.S. Pat. No. 3,892,905, at Col. 3, line 47 Col. 4, line 4 and Col. 4, lines 44-54. Many of the prior art films suffer from poor aging stability when in contact with alkaline substances, even under moderate (e.g., 50% R.H.) humidity conditions. For example, they may become water insoluble after only 2 to 3 weeks.
Polyvinyl alcohol is not fully compatible with most polymers and, therefore, does not form completely homogeneous blends and, accordingly, it is difficult to form uniformly transparent and mechanically strong films. Still further, the prior art films do not have sufficiently high rates of dissolution in water, particularly cold water, i.e., at water temperatures below about 60.degree. F., especially at or near ice water conditions.